1. Technical Field
The present invention relates generally to modems for very small aperture terminal satellite communications systems and, more particularly, to modems for improved direct sequence spread spectrum communication systems.
2. Discussion
The term VSAT stands for very small aperture terminal. VSAT systems typically support satellite communications networks over a two-way communication link. VSAT systems may have particular applicability to banking and financial institutions, airline and booking agencies, and retail stores which cover large geographical areas. VSAT systems have facilitated the growth and connectivity of a worldwide telecommunications infrastructure. Particularly, VSAT systems may connect remote and undeveloped regions with modern communication facilities of major cities, via commercial satellite communication links.
A typical VSAT network includes a hub station that provides network control for all VSATs in the network. A VSAT system may include individual VSATs that access communication satellite transponders using a variety of multiple-access modes. In a typical link from a hub to a VSAT, the hub terminal broadcasts a signal for reception by all VSATs in the network. The outlying VSAT terminals will transmit to other VSATs or the hub terminal using established protocols.
A variety of communication modes may be used to establish connectivity from individual VSAT terminals to the hub. Access methods include code division multiple access (CDMA), frequency division multiple access (FDMA), and time-division multiple access (TDMA). The traffic in a VSAT network is typically data, transferred in packets or bursts, such as in an inventory control system, that occur at random and possibly infrequent intervals. One data transfer method for frequency re-use is direct sequence spread spectrum (DS-SS) code division multiple access (CDMA).
In CDMA, each signal is assigned a unique pseudonoise (PN) code that spreads the signal spectrum. While all signals may be received simultaneously by a receiver, a unique PN code enables recovery of the desired signal by correlating the PN code with the received signal. Other signals occupying the transponder channel appear as random noise to the spread spectrum demodulator. In a typical CDMA application, the code rate must be much greater than the traffic rate and is purposely chosen to spread the signal spectrum over the available transponder bandwidth.
An exemplary spread spectrum implementation, as applied to CDMA, will employ a large number of mutually low cross-correlation pseudorandom codes. A pseudorandom code will have some characteristics of a purely random sequence, but generally has an underlying deterministic structure. A typical pseudorandom code comprises a finite-length binary sequence which is used to phase modulate the carrier. By providing a copy of the pseudorandom code to the receiver, the spread spectrum signal may be correlated with this code and despread, to recover the baseband signal or underlying data.
The finite-length binary sequence has many of the properties typically associated with wideband noise, and thus is often referred to as pseudonoise (PN). The binary sequence is nominally generated by a linear or nonlinear feedback shift register generator. The PN bits are commonly referred to as chips, to avoid confusion with the bits in the underlying baseband signal or traffic.
The PN sequence or code generator is typically clocked at a uniform rate which exceeds the underlying data bit rate or traffic data rate by some constant factor, related to the processing gain of the spread spectrum system.
Spread spectrum signal systems may be configured to reject intentional and unintentional jamming by interfering signals so that information can be communicated, particularly in some military scenarios. Sustained performance of mobile or ground communication terminals in a severe interference or jamming environment is a particularly attractive feature of spread spectrum communication. Many of these systems also employ direct sequence spread spectrum techniques to provide a measure of covertness in a high surveillance environment.
Spread spectrum signals have a low probability of intercept or detection because the power in a transmitted signal is spread over a large bandwidth or frequency space. Further, such signals cannot be readily detected without knowing certain signal parameters, thereby ensuring message privacy. The ability to send many orthogonal signals over the same frequency band provides a frequency re-use feature for CDMA spread spectrum signals. Certain terrestrial wireless local area network and cellular communications systems employ CDMA and spread spectrum techniques to support cellular communications.
Many spread spectrum communication systems utilize conventional quadrature phase shift keying (QPSK) modulation schemes which utilize both the in-phase (I) and quadrature phase (Q) channels to convey information. While such a modulation scheme may be efficient from a power perspective, a conventional direct sequence spread spectrum signal must acquire the PN codes at signal to noise (SNR) ratios close to those employed for communications. Such a requirement results from nearly equivalent I channel and Q channel processing gains.
There is a need for a more precise signal parameter measurement system used in conjunction with a QPSK spread spectrum system. Such a system enables improved acquisition and signal tracking performance when the quality of the communication link is poor or where minimum power levels are preferably used to implement the communication.
The present invention is directed to spread spectrum modem apparatus including a modulator for modulating data and spread spectrum code information onto a spread spectrum signal. The spread spectrum signal has a first channel component and a second channel component. Data and a first spread spectrum code is modulated onto the first channel component, and a second spread spectrum code is modulated onto the second channel component. A demodulator receives the spread spectrum signal and acquires the second spread spectrum code in the second channel component and acquires the first spread spectrum code based upon the acquired second spread spectrum code information. The demodulator determines the data by despreading the spread spectrum signal based upon the acquired first and second spread spectrum code information.